On Thu, Aug 01, 2002 at 05:37:46PM -0700, Andrew Morton wrote:
> This is a large-page support patch from Rohit Seth, forwarded
> with his permission (thanks!).
Overall, the code looks very clean.
(1) So there are now 4 of these things. How do they compare to each
other? Where are the comparison benchmarks? How do their
features compare? Which one(s) do users want?
(2) The allocation policies for pagetables mapping the things may as
well do some kind of lookup, sharing, and cacheing; it's likely
a significant number of the users of the shm segment will be
mapping them more or less the same way given database usage
patterns. It's not a significant amount of space, but kernels
should be frugal about space, and with many tasks as is typical
of databases, the savings may well add up to a small but
respectable chunk of ZONE_NORMAL.
(3) As long as the interface is explicit, it might as well drop flags
into shm and mmap. There isn't even C library support for these
things as they are... time to int $0x80 again so I can test.
(4) Requiring app awareness of page alignment looks like an irritating
porting issue, which doesn't sound as trivial as it would
otherwise be in already extremely cramped 32-bit virtual
(5) What's in it for the average user? It's doubtful GNOME will be
registering memory blocks with these syscalls anytime soon.
Granted, the opportunities for reducing TLB load this way
are small on desktop systems, but it doesn't feel quite
right to just throw mappings of magic physical memory into the
hands of a few enlightened apps on machines with memory to burn
and leave all others in the cold.
By several accounts "scalability" is defined as "performing as
well on large machines as it does on small ones" ... but this
seems to be a method of circumventing the kernel's own memory
management as opposed to a method of improving it in all cases.
(6) As far as reconfiguring, I'm slightly concerned about the robustness
of change_large_page_mem_size() in terms of how likely it is to
succeed. Some on-demand defragmentation looks like it should be
implemented to make it more reliable (now possible thanks to
rmap). In general, the sysctl seems to lack some adaptivity.
Granting root privileges to the workload vs. perpetual
monitoring to find the ideal pool size sounds like a headache.
(7) I'm a little worried by the following:
zone(0): 4096 pages.
zone(1): 225280 pages.
BUG: wrong zone alignment, it will crash
zone(2): 3964928 pages.
My machine doesn't seem to care, but others' might.
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This archive was generated by hypermail 2b29 : Wed Aug 07 2002 - 22:00:18 EST